The resistance value provided by a resistor is a function of the resistivity of the material of which it is made and its geometry, in particular its cross-sectional area and its length; if the material's resistivity is considered to be a fixed value, the resistance value provided by a resistor is directly proportional to its length and inversely proportional to its cross-sectional area.
Providing resistors on integrated circuit chips which have predetermined fixed resistance values can be fairly difficult, as manufacturing process variations makes it difficult to ensure that resistors will have precise geometries, and in any event the resistivity of the material from which the resistors are made generally varies with the operational temperatures over which the chip may be used. While it is difficult to provide resistors on an integrated circuit chip which have predetermined values, it should be noted that resistance values of resistors on an integrated circuit chip will vary generally proportionally, so that, while absolute resistance values of resistors may vary, ratios of resistance values will generally remain constant. Accordingly, most of the circuits on an integrated circuit chip are designed to avoid the necessity of having resistors with particular resistance values, but instead circuits are designed so as to use ratios of resistance values.
However, in some circuitry on a chip, ratios of resistance values generally will not suffice. This is particularly the case in circuitry used in input/output circuitry, that is, circuitry used to receive signals from, or transmit signals to, circuitry external to the chip. Generally, such input/output circuit is expected to accommodate certain signalling protocols, in particular to accommodate certain voltage levels and to provide predetermined impedance values. This generally requires providing resistors having selected absolute resistance values.